FR2581818A1 - REGULATION CURRENT POWER SUPPLY FOR VIDEO VISUALIZATION DEVICE - Google Patents

Abstract

The invention relates to a regulating current supply for a video display device. According to the invention, there is provided a switching transistor 15 which periodically energizes a transformer winding 13; input switching pulses for transistor 15 vary in width and frequency to optimize current transfer through transformer 14; the charge and discharge patterns of a capacitor 36 determine the width and frequency of the pulses; the capacitor charge current is selectively switched to be applied to capacitor 36 while the capacitor discharge current is monitored in response to a feedback signal derived from the regulated voltage level. The invention applies in particular to the power supply of televisions or computer monitors. (CF DRAWING IN BOPI)

Description

The present invention relates to food

  Switched-type control current modes for video display device and, in particular, switched type power supplies that operate in response to pulses of varying width and frequency. In video display devices such as televisions or computer monitors, power supplies are incorporated which produce one or more regulated voltage levels which are used to power various load circuits of the video display device. Such a power supply utilizes a switching device, such as a transistor, which periodically energizes a primary winding of a transformer of an unregulated voltage source in response to trigger pulses. The width of the trip pulse is controlled so that the secondary winding of the transformer develops a voltage that remains constant regardless of the changes and variations of the load in the unregulated voltage level. Variations in power requirements for a load circuit, as may occur with a circuit incorporated in a video display device, may force variations in

  pulse width of the current supply to modula-

  tion of the pulse width previously described in

  to be quite broad. Wide variations in pulse width

  Consequently, the result can be significant variations in the flux density in the winding of the transformer, thus increasing the

  complexity of circuit and transformer design.

  A power supply having a regulating arrangement of pulse frequency modulation with a fixed pulse width simplifies the design and construction of the transformer but may introduce other difficulties. For example, at very low load conditions, the pulse frequency may decrease in the audible range. Under high load conditions, insufficient current can be transferred to maintain

  regulation of the voltage level.

  According to one aspect of the present invention, a power supply for a video display device comprises an unregulated voltage source, a transformer winding, and a switch for coupling the voltage source to the winding in response to pulses of power. Entrance. A pulse generator

  forms a current source that produces a level of

  predetermined amount. A first current path that

  contains a capacitor is connected to a second trajec-

  current field which contains a control circuit. A circuit responsive to the voltage level across the capacitor couples the current source to the first and second current paths when the capacitor is discharged below a first voltage level and decouples the current source from the first and second current paths. when the capacitor is charged above a second voltage level. A circuit is coupled to the control circuit of the second current path and responds to a reference voltage to control current flow through the second current path. The second current path, in a first mode, diverts current from the current source away from the capacitor to control the capacitor charge time and, in a second mode, discharges the capacitor. The control circuit controls the time of

discharge of the capacitor.

  The invention will be better understood, and other purposes, features, details and advantages thereof

  will become clearer during the description

  explanatory text which will follow with reference to the appended schematic drawings given solely by way of example and illustrating an embodiment of the invention and in which: FIG. 1 gives a partially block diagram of part of a video display device is incorporated a power supply according to one aspect of the present invention; FIG. 2 illustrates waveforms useful for understanding the circuit of FIG. 1; and

  FIG. 3 is a diagram of an embodiment of

  of a pulse-forming circuit shown on the

figure 1.

  Referring to FIG. 1, it shows part of a video display device such as a television or a computer monitor, where an AC voltage of an AC voltage supply of the network 10 is applied to a circuit Rectifier II and a filter capacitor 12 to produce an unregulated DC voltage. The unregulated DC voltage is applied to a terminal of a primary winding 13 of a transformer 14. The transformer 14 may be

  for example, to produce electrical isolation between

  and the user access terminals (not shown) as audio and video input and output terminals. The isolation is achieved by electrically isolating the video and audio charging circuits, for example from the power supply 10 via the transformer 14. The other terminal of the primary winding 13 is coupled to the drain of a MOSFET

  (metal-field effect transistor - semi-oxide

  conductive) which according to one aspect of the present invention receives gate input pulses of variable width and frequency produced in a manner to be described hereinafter. The gate input pulses force the MOSFET 15 to change conduction states in a manner that produces a regulated voltage in the secondary winding 16 of the transformer 14. Additional secondary windings may be provided to produce other regulated voltage levels that can be used to power various load circuits of the video display device. The voltage produced in a sense winding 34 is rectified and filtered to produce a voltage level at a terminal 35 which indicates the regulated voltage level produced by winding 16. The voltage at terminal 35 applies a feedback to the generator circuit of impulses that control

the switching of the MOSFET 15.

  The voltage developed in the winding 16 is rectified and filtered by the diode 17 and the capacitor 20 respectively, and is applied to a terminal of a primary winding 21 of a high-voltage transformer 22. The other terminal of the primary winding 21 is coupled to a conventional horizontal deflection circuit 23 which includes a horizontal driver 24 which applies switching pulses to a horizontal output transistor 25. The horizontal deflection circuit 23 also includes a damping diode 26 , a return capacitor 27, a horizontal deflection winding 30 and a S-shaped capacitor 31. The horizontal deflection circuit 23 produces a deflection current in the baffle winding 30 which is placed around the neck of a cathodic ray tube (not shown) to produce the horizontal deflection of the electron beams of the cathode ray tube. Switching of the horizontal output transistor 25 produces horizontal return pulses in the winding 21 which are raised and rectified by the high voltage winding 32 to produce a high voltage level of the order of 30,000 volts at a terminal terminal 33 The high voltage is applied to the final terminal (not shown) of the cathodic ray tube. The high voltage transformer 22 may also include additional windings, such as winding 48, which can develop a voltage that is rectified and filtered to apply current to one or more of the

  video viewing by a terminal 39.

  The output of the MOSFET input pulses occurs as follows. The pulses produced are forced to vary both in pulse width and frequency. The possible variation of the pulse width is smaller than the corresponding variation in a fixed frequency modulated pulse width current supply because the frequency of the pulses may also vary. In the circuit example of FIG. 1, during operation, the pulse width is forced to vary only in a range of about 4 ps to 8 ps while the pulse frequency may

  vary in a range of the order of 5 kHz to 80 kHz.

  During normal operation, the frequency of

  sions tends to vary from 40-50 kHz. The charge and discharge rates of a capacitor 36, shown by the solid line waveform in FIG. 2A, determine the pulse width and frequency. The charging time of the capacitor 36 between two threshold values TH1 and TH2 determines the pulse width while the discharge time of the capacitor 36 between the threshold values TH2 and TH1 determines the time between pulses, which is equal to the pulse frequency. Threshold values TH1 and TH2 are set by flip-flop 37, which may be of the R-S type designed to switch to values

  thresholds that are 1/3 and 2/3 of the supply voltage.

  The voltage developed in the capacitor 36, present at the terminal 40, is applied to the inputs S and R of the flip-flop 37. The Q and output terminal of the flip-flop 37, under the control of the voltage levels at the terminal 40, switches in response to capacitor 36 reaching respective threshold levels during charging or discharging. The Q output of the flip-flop 37, which produces a signal such as that shown by the solid line waveform in Fig. 2B, is applied to a pulse forming circuit 81. The output of the pulse forming circuit 81 , shown by the solid line waveform

  of Figure 2C, is applied to the gate of the MOSFET 15.

  The capacitor 36 is charged by the power supply + V1 via a charge path comprising a resistor 41, a transistor 42 and a transistor 43 of a differential amplifier 44. The input to the gate or gate terminal The base of transistor 43 is connected to a voltage reference source VREF6 which is equal to half of the logic excursion of flip-flop 37, i.e. half of the voltage excursion of the Q output. The gate or base electrode of the transistor 45 of the differential amplifier 44 is coupled to the Q output of the flip-flop 37. The discharge path of the capacitor 36 comprises a transistor 46 and a resistor 47. The charging current through the resistance 41 and the

  transistor 42 represents a known level of current deter-

  undermined in a way that will be explained below. This current is led by the transistor 43 or the transistor 45, determined by the state of the level at the input terminal 49 of the transistor 45, which is the output Q of the flip-flop 37. The degree of conduction of the transistor 46 will determine the

  charge and discharge rates of the capacitor 36.

  While transistor 43 is conducting, transistor 46 controls the diversion of the charging current away from capacitor 36, which increases the charging time. The maximum current diverted by the transistor 46 is set, in a manner to be described, to be equal to half the charging current so that the transistor 36 continues

  to charge as long as the transistor 43 is conducting.

  When the transistor 43 is not conducting, the conductor

  transistor 46 forces the capacitor 36 to

  discharging. Therefore, the degree of conduction (ie

  ie the current flow) of transistor 46 will determine

  both the charging and discharging

  36 and therefore the width and frequency of the pulses applied to the MOSFET 15. The voltage level at the base of the transistor 46, which controls the degree of

  conduction of this transistor, is determined by an amplification

  Differential encoder 50, essentially comprising transistors 51, 52, 53 and 54 and a current source transistor 66. The input of the transistor 54, an input of the differential amplifier 50, is derived from a carefully controlled reference voltage VREF1. The other input of the differential amplifier 50, the transistor 51, receives a regulator feedback signal from the terminal 35 which is derived from the voltage developed in the sense coil 34 which, in turn, is derived from the regulated voltage produced by the transformer 14. While the reaction voltage decreases with respect to VREF1, indicating a fall in the regulated voltage level, the transistor 52 becomes more conductive which, by the transistor 58 connected diode, raises the voltage to the base of the transistor 46, which forces the transistor 46 to become more conductive. This results in the capacitor 36 charging at a slower pace, as shown in the dotted waveform of FIG. 2A, thereby increasing the width of the pulse at the input of the MOSFET, as shown by FIG. dotted waveform in FIG. 2C, when the

  charge current is turned on by the transistor 43.

  When the charging current is turned off and the current flows through the transistor 45, an increased conduction of the transistor 46 forces the capacitor 36 to be discharged more rapidly, thereby increasing the frequency of the pulses at the input of the transistor. MOSFET as can also be seen in Figure 2C. MOSFET 15 will therefore be conducting longer and more frequently, as shown in the dotted waveform of FIG. 2D,

  forcing the regulated voltage level to increase. Conversely

  an increase in the level of regulated voltage caused by

  increase the voltage applied to the transis-

  tor 51, making it less conductive, forcing the transistors 52 and 46 to be less conductive, thereby decreasing the width and frequency of the pulses so that the regulated voltage decreases. The circuit operates in this way by simultaneously varying the pulse width and frequency to maintain the voltage

regulated at a constant level.

  As previously described, the charging current supplied by the transistor 42 and the transistor 43 is of a known value, with the maximum discharge current flow through the transistor 46 referenced on the charging current in the same manner. next. Differential amplifier 60, comprising transistors 61, 62 and 63, has an input, the base of transistor 62, which is coupled to a reference voltage VREF2. The conduction of transistor 62 causes its collector voltage to drop, thereby the transistor 64 being more conductive, since its base is connected to the collector of the transistor 62. The increased conduction of the transistor 64 increases the flow of current through the resistor 65, which increases the voltage drop across the resistor 65, thereby raising the voltage at the base of transistor 61, making it more conductive. The increased conduction of the transistor 61 decreases the conduction of the transistor 62 so there is a decrease in the conduction of the transistor 64 and consequently of the transistor 61. In this reactive manner, the voltage at the base of the transistor 61 is maintained substantially equal to the voltage at the base of transistor 62, which is equal to VREF2. Since the voltage at the collector of the transistor 64 is known, the choice of the desired value for the resistor 65 determines the flow of current through the transistor 64. The base of the transistor 64 is also coupled to the bases of the transistors 42 and 66, so their flow of current is also determined. The transistor 42 is conducting the charging current through the resistor 41 while the transistor 66 controls the

  current of decharge of the transistor 46 by a resistor 67.

  Therefore, the ratio of the values of the resistors 41 and 67 will determine the maximum ratio of the charging current to the discharge current. By way of example, the charging current is chosen to be twice the discharge current at the maximum conduction of the transistor 52. Under these conditions, this results in equal currents charging and discharging the capacitor 36, thereby producing pulses. at the entrance of the MOSFET having a useful life of 50%, which is desirable for the best

power transfer.

  During the start-up operation of the food

  when the video display device is initially excited, for example, it is desirable to limit the width and frequency of

  the impulse produced to allow the supply voltages to

  circuit to increase to their normal levels without undue burden. A slow start circuit is provided which allows pulse width and frequency

  to increase slowly. When the visualization device

  The video circuit is stopped, the logic circuit 70 causes the transistor 71 to be momentarily conducting, thereby discharging the capacitor 72. When the video display device is energized, the capacitor 72 is discharged but begins to charge slowly increasing power. V1 through resistor 73, which forms a voltage divider with resistor 74. With capacitor 72 discharging, the voltage level at the base of transistor 75 turns it on, thereby turning on

  the transistor 76 by the transistor 77 connected diode.

  Transistor 76 will divert or conduct the discharge control current of transistor 52 such that the pulses produced at the input of the MOSFET have narrow widths and a low frequency. As the + V operating power increases, the voltage developed in the capacitor 72 causes a decrease in the conduction of the transistor 75, and therefore a decrease in the conduction of the transistor 76. The discharge control current then begins to control the

  voltage at the base of transistor 46 so that the pulses

  are produced become larger and more frequent.

  When the operating power + V1 reaches

  its normal level, the voltage in the condensa-

  72 causes the transistors 75 and 76 to become non-conductive, thereby terminating the slow start interval and allowing the pulse generator to operate.

in its normal way.

  The logic circuit 70 can also be used to inhibit the power supply, and therefore the video display device, under certain defective conditions. For example, a conductor 80 applies an excess current detection circuit to the

  logic circuit 70 of the MDSFET 15. In the case of a

  In the case of excess current, the increased voltage drop across the resistor 79 is applied by the conductor 80 to the logic circuit 70 to make the transistor 71 conductive, which discharges the capacitor 72.

  conduction of transistor 76 which effectively inhibits the

  mentation. Other defective conditions

  can also force the logic circuit 70 to function

  in a similar way so that the video display device can not operate during

defective conditions.

  The output of the flip-flop 37, shown in FIG. 2B, is a slot pulse corresponding to the desired switching signal for the MOSFET 15. In order to reduce the possible high frequency interference problems (rfi) that can be caused by applying a pulse At high pitch at MOSFET 15, a pulse forming circuit 81 is provided which processes the signal at the output of flip-flop 37 to produce a signal comprising pulses having controlled rise and fall times as shown in FIG. 2C. . One embodiment of the pulse forming circuit 81 is shown in FIG. 3. The output signal, at the output Q of the flip-flop 37, is

  applied to the terminal 82 which is the base of the transistor 83.

  Positive pulses at the output of the flip-flop 37

  turn on transistor 83, which, by

  Current flow through diode 84 and resistor 85 raises the voltage at the base of transistor 86, turning it on. The conduction of transistor 86 discharges capacitor 87 which is usually charged by a constant current source including transistor 90 and resistor 91. Diodes 92 and 93 block the level at which capacitor 87 can charge. The discharge rate of the capacitor 87 is determined by the values of the resistors 85, 94 and 95. The discharge of the capacitor 87 forces the voltage at the base of the transistor 96 to decrease, thus reducing its conduction and forcing the

  current through the resistors 97 and 100 to decrease.

  While the voltage across the resistor 100 decreases, the conduction of the transistor 101 decreases resulting in a linear increase in the voltage across the resistors 102 and 106. The voltage increase is linear because a relatively small portion of the charge voltage characteristic RC of the capacitor 87 is amplified by the transistors 96 and 101. The emitter voltage of the transistor 103 also increases linearly to a level equal to the power supply level + V1 minus the low-emitter drops transistors 104 and 105. The emitter of transistor 103 is coupled to the base of the MOSFET 15 and produces the pulses

  actual switching of the MOSFET 15.

  Negative flanks of pulses from flip-flop 37 to terminal 82 turn transistors 83 and 86 off

  circuit. the capacitor 87 is then charged by the power supply

  + V1 via resistor 91 and transistor 90.

  This causes an increase in the conduction of

  The result is a linear decrease of the voltage across the resistor 102. The conduction of the transistor 103 increases, so the emitter voltage and therefore the pulse at the input of the MOSFET decrease linearly. The choice of values for the resistors 85 and 95 determines the rise time of the pulses while the choice of the resistance 91 determines the time of

fall of the impulse.

  The logic circuit 70 applies, by way of example, a signal to the terminal 110 of the pulse forming circuit 81 during an excess current condition so that the MOSFET 15 remains off, thereby producing a

  further inhibition of the power supply.

  As shown in FIG. 3, the terminal 110 is connected to the collector of the transistor 101.

R E V E N DI C A T I ONS

  1. Power supply for a video display device of the type comprising: an unregulated voltage source; a transformer winding; switching means for coupling said unregulated voltage source to said transformer winding in response to input pulses; and a pulse generator coupled to said switching means, characterized by: a current source (V1, 41, 42) producing a predetermined level of current; a first current path (43, 36) including a capacitor (36); a second current path (43,46) including control means (46) coupled to said first current path (43,36); means (37,44) responsive to the voltage level across said capacitor (36) for coupling said current source (V1,41,42) to said first (43,36) and second (43,46) current paths when said capacitor (36) is discharged below a first predetermined voltage level for charging said capacitor (36) and for decoupling said current source (V1, 41, 42) from said first (43, 36) and second (43, 36) , 46) current paths when said capacitor (36) is charged above a second predetermined voltage level; and means (51, 52, 53) coupled to said control means (46) and responsive to a reference voltage level for controlling the amount of current flow through said second current path (43, 46), said second current path (43, 46) diverting the charge current from said capacitor (36) when said current source (Vi, 41, 42) is coupled to said first (43, 36) and second (43, 46) current paths , said control means (46) controlling the charging time of said capacitor (36), said second current path (43, 46) discharging said capacitor (36) when said current source (V1, 41, 42) is decoupled from said first (43,36) and second (43,46) current paths, said control means (46) controlling the discharge time

said capacitor (36).

  2. Power supply for a video display device of the type comprising: an unregulated voltage source; a transformer winding; switching means for coupling said unregulated voltage source to said transformer winding in response to input pulses; and a pulse generator coupled to said switching means, characterized by: a source (41,42) of current producing a predetermined level of current; a first current path (43, 36) including a capacitor (36); a second current path (43, 46) comprising current control means (46) coupled to and forming a shunt for said first

current path (43, 36).

  3. Power supply for a video display device comprising: an unregulated voltage source, a transformer winding; switching means for coupling said unregulated voltage source to said transformer winding in response to the input pulses; and a pulse generator coupled to said switching means; characterized by: a capacitor (36); a discharge current path including current control means (46) coupled to said capacitor (36) for producing a discharge path of said capacitor (36); and a charge current path containing a charge current source (41, 42) selectively coupled to said capacitor (36) for producing a charge path of said capacitor (36), a portion of said charge current being diverted

  by said discharge current path (46).

  4. Power supply according to the claim

  3, characterized in that the current which is diverted is smaller than the current supplied by the source (41, 42).

aforementioned current.